FIG. 6 shows a conventional cathode for an electronic tube used in cathode ray tubes in televisions and image pickup tubes as disclosed in Japanese Patent Laid-open No. 3-257735.
In FIG. 6, 1 is a substrate mainly composed of nickel (Ni) containing a small amount of reduction elements such as silicon (Si) and magnesium (Mg), 2 is a cathode sleeve composed of chromium and the like, 13 is a metal layer mainly composed of at least one or more of tungsten, molybdenum, tantalum, chromium, silicon and magnesium, and 5 is a layer of an electron-emitting material deposited on the metal layer 13, containing at least barium in addition to alkali earth metal oxides 11 which include strontium and/or calcium as main components and also containing 0.1 to 20% by weight of rare earth metal oxides 12 such as scandium oxide, and 3 is a heater disposed within the substrate 1 that discharges thermoelectrons from the electron-emitting material 5 by heating.
Due to the construction of the conventional cathode for electronic tubes described above, when a metal layer of tungsten is formed on the substrate, tungsten in the metal layer diffuses into the substrate mainly composed of nickel by heat aging and emission aging during the CRT production process or by the heat generated during its operation. While this diffusion layer usually forms as a Ni--W layer with a thickness of 10 to 20 microns after 10,000 operating hours, the substrate is thermally deformed into a convex or concave shape depending on the thickness of the tungusten metal layer at the initial stage of life of the cathode because the thermal expansion coefficients of the layer of the electron emitting material of the substrate and the layer close to a heater differ, or because layers having different lattice constants are separately formed. A problem arose due to this deformation in that the distance between the first grid (electrode) and the surface of the cathode fluctuates, resulting in a drift of the cutoff voltage. This drift in the cutoff voltage also caused a problem in that after a prolonged operating time, the brightness of the screen greatly fluctuates or, in the case of color cathode ray tubes, the hue is greatly fluctuates.
The object of the present invention is to solve the foregoing problem by providing a method for reducing the thermal deformation of the substrate to obtain a cathode for electronic tubes with a small drift of the cutoff voltage.